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Wind Shear

The environmental wind field is also very important in determining what
type of a thunderstorm could develop.
Severe weather usually occurs when
the change in horizontal winds (wind shear) is significant. This includes
the change in wind direction (directional shear) or speed (speed shear).

Speed Shear
In speed shear, the wind
increases in speed from the surface to the upper levels, as shown in
this diagram by the arrows. This vertical shear creates horizontal
rotation which can best be visualized by placing a paddle wheel in the
environment. Besides the rotation, the change in wind speed with height
lets the updraft separate from the downdraft,
allowing the storm to survive longer and even become stronger.

Directional Shear
Directional shear refers to a change in the direction of the wind with
height. Notice in this image there is both speed and directional shear
as both the angle and the length of the wind vectors are changing with
height. Two viewpoints are shown in the following figure. The length
of the arrows represents the wind speed. The arrows point in the
direction that the wind is blowing and are located at different
heights in the column of air shown. Both speed and directional changes
occur from one level to another.